1. Field
This disclosure relates generally to touch sensors, and more specifically, to a touch sensor controller for driving a touch sensor shield.
2. Related Art
Touch sensors have been available for a number of years as a relatively inexpensive alternative to mechanical switches. One type of touch sensor, commonly known as a capacitive touch sensor, or pad, detects a change in capacitance when an object, such as a human finger touches the pad. In an application such as a telephone touch pad, a number of capacitive touch pads are arranged as an array, or matrix, of touch pads that are spaced apart and arranged as the telephone shape, size and design allows.
A touch sensor controller is used to control operation of the touch sensor and provides detection circuitry for determining when a pad is touched. As part of the function of the detection circuitry, a baseline capacitance may be established on each pad. Touching a pad changes the capacitance and the change is detected by the touch sensor controller. A touch sensitivity of the pads is established, as least in part, by the baseline capacitance. An increase of the baseline capacitance can decrease sensitivity.
There are many environmental and design conditions that can affect the baseline capacitance of the capacitive touch pads. For example, the baseline capacitance of the pads can be increased by the presence of a person's hand near the touch pads. Shielding around the touch pads can be used to reduce the effects of stray capacitance. Typically, the shielding is formed as a ring around each pad, or as a plane or casing on the backside of a printed circuit board (PCB) supporting the pads, or both. The shielding may be grounded. A potential difference between the pads and the shield may result in a parasitic capacitance between the shield and the pads, thus changing the baseline capacitance and reducing touch sensitivity.
Therefore, what is needed is a touch sensor controller that solves the above problems.